The invention relates to a thermoelectric element.
The manner in which thermoelectric elements function relates to the thermoelectric effect:
By virtue of the thermoelectric effect, also referred to as the Seebeck effect, an electric voltage is produced between two points of an electrical conductor or semiconductor, said two points having a different temperature. The Seebeck effect describes the reversible alternating effect between temperature and electricity. The Seebeck voltage is determined by means of:USeebeck=α×δT where:                δT represents the temperature difference between the hot face and the cold face,        α represents the Seebeck coefficient or rather the thermoelectric power.        
The Seebeck coefficient is a measurement of the magnitude of an electric voltage per temperature difference (V/K). The magnitude of the Seebeck coefficient is substantially responsible for the magnitude of the Seebeck voltage.
The thermoelectric elements are embodied preferably from differently doped semiconductor materials, as a consequence of which it is possible to increase considerably the efficiency in comparison to thermoelements that are embodied from metals. Conventional semiconductor materials are, for example, Bi2Te3, PbTe, Bi2Se3, SiGe, BiSb or FeSi2.
Whereas the Seebeck effect describes the production of a voltage, the Peltier effect occurs exclusively as a result of the flow of an external current. The Peltier effect occurs if two conductors or semiconductors that have different electronic thermal capacities are brought into contact with one another and electrons flow from one conductor/semiconductor to the other as a result of an electric current. Using suitable materials, in particular semiconductor materials, it is possible to use the electric current to produce temperature differences or, conversely, to produce electric current from the temperature differences.
In order to obtain sufficiently high voltages, several thermoelectric elements are combined to form a thermoelectric module and are connected in series in an electrical manner and where appropriate are also connected in parallel.
A thermoelectric Peltier module illustrated in FIG. 1 comprises several series-connected thermoelectric elements. The thermoelectric elements (1) comprise in each case small rectangular blocks (2a, 2b) that are embodied from p-doped and n-doped semiconductor material and are provided alternately above and below with metal bridges (3a, 3b). The metal bridges (3a, 3b) form the thermal and electrical contacts of the thermoelectric elements (1) on a, hot or cold face (4, 5) respectively of the thermoelectric module and are mainly arranged between two ceramic plates (6a, 6b) that are arranged at a distance from one another. The differently doped rectangular blocks (2a, 2b) are connected to one another by means of the metal bridges (3a, 3b) in such a manner that they produce a series connection.
Insofar as an electric current is supplied to the rectangular blocks (2a, 2b), the connection sites of the rectangular blocks (2a, 2b) on one face (4, 5) cool down and the connection sites of the rectangular blocks (2a, 2b) on the opposite face (4, 5) warm up in dependence upon the current strength and the current direction. Consequently, the applied current produces a temperature difference between the ceramic plates (6a, 6b). If, however, the temperature prevailing at the opposite-lying ceramic plate (6a, 6b) is different, a current flows into the rectangular blocks (2a, 2b) of each thermoelectric element (1) of the module in dependence upon the temperature difference.
The edge length (7) of the rectangular blocks (2a, 2b) perpendicular to the ceramic plates (6a, 6b) amounts to approx. 3-5 mm. The long edge length (7) requires a high thermal resistance between the hot and cold face (4, 5), so that the Seebeck voltage and the output of the module is greater in comparison to a Peltier module that is illustrated in FIG. 2 and that comprises a shorter edge length (7) of the rectangular blocks (2a, 2b) but with an identical cross section of the rectangular blocks (2a, 2b). However, the rectangular blocks (2a, 2b) that have the longer edge length (7) require more semiconductor material.
The conversion efficiency of the conventional, above-mentioned thermoelectric materials is currently in the range below 5%. This means that the heat flow must amount to more than 20-times the required electrical output. Since the specific thermal conductivity of the conventional, above-mentioned thermoelectric materials is in the range of 1-5 W/mK, the specific thermal conductivity of the thermal contacts of the rectangular blocks must, be considerably above 20-100 W/mK.
The heat flow in the rectangular blocks reduces in the case of an identical cross section of the rectangular blocks (2a, b) as the edge length m increases. The achievable thermal resistances are therefore only dependent upon the specific thermal conductivity and the edge length (7) of the rectangular blocks (2a, b). It is therefore even more difficult to supply heat to thermoelectric elements of the type illustrated in FIG. 2.